Method and apparatus for setting black level in an imager using both optically black and tied pixels

ABSTRACT

An imaging pixel array includes an active area of pixels, organized into rows and columns of pixels. The array also includes a plurality of dark pixel columns adjacent to the active area of pixels such that rows of pixels in the active area of pixels extend across the plurality of dark pixel columns. The plurality of dark pixel columns are composed of tied pixels. The array also includes a plurality of dark pixel rows adjacent to the active area of pixels and the plurality of dark pixel columns such that columns of pixels in the active area of pixels extend across the plurality of dark pixel rows. The plurality of dark pixel rows are composed of both optically black pixels and tied pixels on the same row.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/302,124, filed on Dec. 14, 2005, now U.S. Pat. No. 7,427,735 thesubject matter of which is incorporated in its entirety by referenceherein.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor imagers. Morespecifically, the present invention relates to noise reduction andsuppression of unwanted artifacts in semiconductor imagers.

BACKGROUND OF THE INVENTION

Complementary metal-oxide semiconductor (CMOS) image sensors utilizesensor arrays that are composed of rows and columns of pixels. Thepixels are sensitive to light of various wavelengths. When a pixel issubjected to a wavelength of light to which the pixel is sensitive, thepixel generates electrical charge that represents the intensity of thesensed light. When each pixel in the sensor array outputs electricalcharge based on the light sensed by the array, the combined electricalcharges represent the image projected upon the array. Thus, CMOS imagesensors are capable of translating an image of light into electricalsignals that may be used, for example, to create digital images.

Ideally, the digital images created through the use of CMOS imagesensors are exact duplications of the light image projected upon thesensor arrays. However, various noise sources can affect individualpixel outputs and thus distort the resulting digital image. Some noisesources may affect the entire sensor array, thereby requiring frame-widecorrection of the pixel output from the array. One such correctivemeasure applied to the output of the entire sensor array is the settingof a base-line black level (described below). Other noise sources mayonly affect specific portions of the sensor array. For example,row-specific noise may be generated from a mismatch of circuitstructures in the image sensors due to variations in the manufacturingprocesses of integrated circuits. The effect of row-specific noise in animage sensor is that rows or groups of rows may exhibit relativelydifferent outputs in response to uniform input light.

In order to set a corrective black level and remove the effects ofrow-specific noise, dark rows and dark columns are used in imagesensors, as demonstrated in FIG. 1. FIG. 1 show an image sensor 100 thatincludes a pixel array 110 organized into N pixel columns and R pixelrows. The pixel array 110 contains an active area 112, dark rows 115 anddark columns 117. Although not shown in FIG. 1, dark rows 115 may alsobe located above the active area 112, and dark columns 117 may also belocated to the left of the active area 112. Upon readout of a row,parallel pixel outputs from each column (i.e., N pixel outputs) aresampled and stored on a set of capacitors 120, one row at a time. Eachpixel is in turn sent through an analog signal processing block 130before being digitized by an analog-to-digital converter 140. The streamof digitized pixels are then processed digitally (block 150) before theyare sent to an output buffer 170. By monitoring the digitized data fromthe dark rows 115, a feedback loop 160 is used to adjust the frame-wiseblack level. Generally, noise reducing processes (block 150) are appliedto each pixel output, sequentially, either before or after the outputreaches the analog-to-digital converters 140.

Dark columns 117 and dark rows 115 are areas within the pixel array 110that do not receive light or capture image data. Pixel outputs from thedark rows 115 and dark columns 117 are used to both set the black levelfor the entire pixel array 110 and correct row-specific noise.

One corrective technique is to ensure that pixels in the dark columns117 and dark rows 115 do not receive image data by covering the pixelsin the dark columns 117 and dark rows 115 with a metal plate. Pixelsblocked from sensing light via a metal plate are referred to asoptically black pixels. Because, theoretically, no light is sensed bythe optically black pixels, the only charge generated by the opticallyblack pixels is internal noise-induced charge. This is often referred toas dark current. Thus, one method of compensating for noise is throughthe calculation of average optically black pixel output values, whichrepresent average noise values, and then subtracting these averagevalues from the outputs of the pixels in the active area 112. Forexample, an appropriate black level may be set by calculating an averageoptically black pixel output for the optically black pixels in the darkrows 115 (block 150), and then subtracting this average value from theoutput of every pixel in the active area 112 and dark columns 117.Row-specific noise in pixel array 110 may also be compensated for bycalculating an average optically black pixel output for each row ofoptically black pixels in the dark columns 117 (block 150). Thecalculated optically black pixel average for each row is then subtractedfrom the values of each of the active pixels in the corresponding rowsof pixels.

In practice, because each row of pixel outputs in the pixel array 110 isread-out sequentially, the pixel outputs from the dark rows 115 are readfirst. From the dark rows 115, the optically black blacklevel iscalculated (block 150) and then applied to the successive pixel outputsfrom the active area 112 and the dark columns 117. Row-specific noise isthen corrected by using the already adjusted optically black pixeloutput values from the dark columns 117. The output values of theoptically black pixels for a given row in the dark columns 117 areaveraged (block 150), and then the averaged optically black pixel outputis subtracted from the output of each of the pixels within therespective row of the active area 112.

A drawback with using optically black pixels in calculating a blacklevel value is that optically black pixels are sensitive to more thanjust background or internal noise. Optically black pixels may generatecharge in response to random, localized noise sources, thus artificiallyaltering the calculated black level. For example, optically black pixelsmay generate excess charge as a result of pixel blooming. Blooming iscaused when too much light enters a pixel, thus saturating the pixel. Apixel subject to blooming is unable to hold all of the charge generatedas a result of sensed light. Consequently, any excess charge may leakfrom the pixel and contaminate adjacent pixels. Optically black pixelsthat generate excess charge as a result of blooming will result in anartificially high black level. Infrared (IR) reflections may also resultin excess charge generation. IR reflections occur when IR radiation isincident on pixels within the pixel array 110 and is trapped within theimage sensor 100. The IR radiation, which also causes pixels to generatecharge, may repeatedly reflect against multiple optically black pixels,thus again artificially inflating the amount of generated charge. Inthese cases, the black level sensed by the optically black pixels isgenerally higher than the ideal black level because of the chargecollected from these noise sources.

In response to the disadvantages of using optically black pixels to setthe black level value, an alternative technique for correcting noise ina pixel array 110 is to tie the photodiode of the pixels in the darkrows 115 to a fixed voltage. The fixed voltage is, in essence, a fixedblack level for the pixel array 110. The advantages of this method isthat the black level calculation is not influenced by blooming, IRreflections, etc., and that every frame utilizes a constant andunchanging black level. However, tied pixels are not sensitive to anychanges in dark current or other row-specific noise sources. Thus, ablack level generated utilizing tied pixels may not accuratelycompensate for the noise caused by dark current.

There is, therefore, a need and desire for a method and apparatus forefficiently generating and applying a stable black level value utilizingthe benefits of tied pixels and optically black pixels to the pixeloutputs of a solid state imager, for example, a CMOS imager.

BRIEF SUMMARY OF THE INVENTION

An imaging pixel array is provided, that includes an active area ofpixels, organized into rows and columns of pixels. The array alsoincludes a plurality of dark pixel columns adjacent to the active areaof pixels such that rows of pixels in the active area of pixels extendacross the plurality of dark pixel columns. The plurality of dark pixelcolumns are composed of tied pixels. The array also includes a pluralityof dark pixel rows adjacent to the active area of pixels and theplurality of dark pixel columns such that columns of pixels in theactive area of pixels extend across the plurality of dark pixel rows.The plurality of dark pixel rows are composed of both optically blackpixels and tied pixels.

Similarly, an apparatus for providing pixel correction values for animager is provided, as well as a method to provide the pixel correctionvalues for the imager. Both the apparatus and method utilize a pixelarray that is composed of an active area, a plurality of dark pixelcolumns and a plurality of dark pixel rows. The plurality of dark pixelcolumns are composed of tied pixels while the plurality of dark pixelrows are composed of a both optically black pixels and tied pixels.

An imager and an imaging system are further provided, both utilizing apixel array that is composed of an active area, a plurality of darkpixel columns and a plurality of dark pixel rows. The plurality of darkpixel columns are composed of tied pixels while the plurality of darkpixel rows are composed of a both optically black pixels and tiedpixels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an image sensor;

FIG. 2 is an image sensor according to an exemplary embodiment of theinvention;

FIG. 3 is an image sensor according to another exemplary embodiment ofthe invention;

FIG. 4 shows the operations of a black level correction circuitaccording to an exemplary embodiment of the invention; and

FIG. 5 is a processor based system that includes an image sensoraccording to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In one exemplary embodiment of the invention, an image sensor utilizesboth tied and optically black pixels to calculate a stable black levelvalue for generated images. FIG. 2 demonstrates an image sensor 200,e.g., a CMOS image sensor, that includes a pixel array 210, a set ofholding capacitors 220, an analog signal processing block 230, a set ofanalog-to-digital converters 240, a set of read-out buffers 270, a blacklevel correction circuit 250 and a feedback loop 260. The pixel array210 includes an active area 212, dark rows 215 and dark columns 217.Within the dark rows 215 are rows 285 of optically black pixels (i.e.,optically black pixel rows 285) and rows 280 of tied pixels (i.e., tiedpixel rows 280). Only tied pixels are in the dark columns 217. Becausethe tied pixel rows 280 are not sensitive to dark current, the outputlevels of the optically black pixel rows 285 and tied pixel rows 280will vary. The variance between optically black and tied pixel outputsmay be compensated for by calculating the difference in readout levelsbetween the optically black pixel rows 285 and the tied pixel rows 280(black level correction circuit 250) and then applying the calculateddifference as an additional black level correction value to the wholeframe (feedback loop 260). For example, circuit 250 could calculate anaverage optically black pixel output or tied pixel output for each rowof the dark rows 215. An average optically black pixel output could thenbe calculated for all optically black pixel rows 285 in the dark rows215, and an average tied pixel output could be calculated for all tiedpixel rows 280 in the dark rows 215. Finally, the difference between theaverage optically black pixel output and the average tied pixel outputcould be calculated. The calculated difference between the average tiedpixel output and the average optically black pixel output is applied asa black level correction value (feedback loop 260).

However, errors may arise when comparing pixel outputs from differentrows. When a row with either optically black or tied pixels is sampledand then average values are calculated for each row, the values areaffected by row-specific noise. The uncertainty introduced byrow-specific noise can be overcome by averaging pixel outputs from asufficient number of multiple optically black pixel rows 285, and thenfinding the difference between this more accurate average opticallyblack pixel output value and an averaged output from a sufficient numberof tied pixel rows 280. To effectively average out any row-specificnoise, however, approximately thirty-two rows per pixel color must besampled. The averaging over thirty-two rows can be accomplished byaveraging a single row sample over thirty-two frames, although adrawback to this method is that when new gain settings are applied tothe sensor, a user must wait for thirty-two frames before a newcorrection factor based on the new gain setting is generated. Apreferable method is to establish the correction value during readout ofthe dark rows so that a new correction factor is computed before thefirst active row is read out. This, of course, suggests the necessity ofintroducing thirty-two physical dark rows to be read and averaged andthen compared with the tied pixel output, thus obtaining a reliablevalue for the black level average for the given gain and integrationtime of the imager. However, the large number of necessary dark rows isundesirable. Not only does the large number of dark rows 215 affect thearea of the pixel array 210 by increasing the array's size and expense,but the increase in rows also affects the frame rate of the imager,since read-out of each frame will take longer.

In an improved exemplary embodiment of the invention, as demonstrated inFIG. 3, an image sensor 300 contains both optically black pixels 385 andtied pixels 380 in each row of the dark rows 315 of a pixel array 310.As depicted in FIG. 3, image sensor 300 also includes a set of holdingcapacitors 320, an analog signal processing block 330, a set ofanalog-to-digital converters 340, a set of read-out buffers 370, a blacklevel correction circuit 350 and a feedback loop 360. The pixel array310 includes an active area 312, dark rows 315 and dark columns 317. Asindicated above, the output from both optically black pixels 385 andtied pixels 380 will vary and must be accounted for. However, in theimproved embodiment of the invention, the difference between opticallyblack and tied pixel output need not account for row-specific noise. Allpixels on a row “see” the same row-specific noise, so values from tiedpixels 380 can be compared with values from the optically black pixels385 without the need for taking many row samples to suppressrow-specific noise. In theory, a single dark row could be sufficient togenerate an accurate black level value. The average output from tiedpixels 380 in the row results in an initial black level value; thedifference between the average tied pixel output and the averageoptically black pixel output in the same row results in an additionalcorrective value. Both the tied pixel value and the additionalcorrective value are calculated by the black level correction circuit350 and are summed together to generate a result which is applied tosucceeding pixel outputs (feedback loop 360), thus setting an accurateblack level. In practice, a few dark rows may be necessary for bothredundancy and to further refine the calculated black level.

The physical organization of tied pixels 380 and optically black pixels370 of dark rows 315 can be a checkerboard pattern, with individualpixels alternating between optically black pixels 385 and tied pixels380. Alternatively, dark rows 315 could be split in the middle, withoptically black pixels 385 on one side of a row and tied pixels 380 onthe other. In the event that rows are split, it is preferable toalternate the sides of the rows whereon the optically black pixels 385and tied pixels 380 are located, so as to facilitate the averaging outof any noise artifacts arising from localized defects of the pixel array310. Any other repetitive pattern of n consecutive tied pixels followedby n consecutive dark pixels could be used, where n is an integergreater than one but less than half the length of dark rows 315. Ingeneral, any symmetrical physical arrangement of tied pixels 380 andoptically black pixels 385 of dark rows 315 is appropriate.

The operations of the black level correction circuit 350 are summarizedin FIG. 4. As pixel values from the dark rows 315 are readout, the blacklevel correction circuit 350 determines the average optically blackpixel value and the average tied pixel value for each row in the darkrows 315 (block 410). For each row, the difference between the averageoptically black pixel value and the average tied pixel value iscalculated (block 420). The processes of blocks 410 and 420 are repeatedfor each of the dark rows 315 (block 430). Once the difference betweenthe average optically black pixel value and the average tied pixel valuefor each row is calculated, an average of the differences is calculated(block 440). The calculated average of the differences is summed with atied pixel value to create an overall black level value (block 450).When pixel values from the active area 312 or dark columns 317 arereadout, black level correction occurs by subtracting the overall blacklevel value from the value of each pixel in the active area 312 and darkcolumns 317 (block 460).

The above-described embodiments of the invention are directed towardssetting an appropriate black level by performing black level correctionprocedures on the analog pixel signal outputs of pixels in a pixelarray. This analog black level correction is implemented by the blacklevel correction circuits 250 and 350 and the feedback loops 260 and 360(FIGS. 2 and 3). However, the black level correction procedures may alsobe applied on digital pixel signal outputs. In this case, the blacklevel correction circuits 250 and 350 act as described, but the feedbackloops 260 and 360 are used to adjust the frame-wise black level afterthe pixel output is digitized. By applying black level correction todigital pixel outputs using noise correction modules 250, 350, the blacklevel correction may be implemented as either a hardware or a softwaresolution. As a software solution, the black level correction may beimplemented as either software integrated with the imagers 200, 300, oras a stand-alone software product stored on a carrier medium andinstalled on a computer system.

A typical processor based system 1000, which includes an imager device1030 according to the present invention is illustrated in FIG. 5. Aprocessor based system is exemplary of a system having digital circuitswhich could include imager devices. Without being limiting, such asystem could include a computer system, camera system, scanner, machinevision system, vehicle navigation system, video telephone, surveillancesystem, auto focus system, star tracker system, motion detection system,or other image acquisition system.

A processor system, such as a camera system, for example, generallycomprises a central processing unit (CPU) 1010, for example, amicroprocessor, that communicates with an input/output (I/O) device 1020over a bus 1090. The imager 1030 also communicates with the systemcomponents over bus 1090. The computer system 1000 also includes randomaccess memory (RAM) 1040, and, in the case of an imaging system mayinclude peripheral devices such as a removable memory 1050 which alsocommunicates with CPU 1010 over the bus 1090. Imager 1030 is preferablyconstructed as an integrated circuit which includes pixels containing aphotosensor, such as a photogate or photodiode. The imager 1030 may becombined with a processor, such as a CPU, digital signal processor ormicroprocessor, with or without memory storage in a single integratedcircuit, or may be on a different chip than the processor.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art(s) that various changes in form and detail can be madetherein without departing from the spirit and scope of the presentinvention. Thus, the present invention should not be limited by any ofthe above-described exemplary embodiments.

1. An imaging pixel array, comprising: an active area of pixels,organized as a plurality of rows and columns of pixels; and a pluralityof dark pixels arranged in a plurality of dark pixel columns adjacent toa first side of said active area of pixels and in a plurality of darkpixel rows adjacent to a second side of said active area of pixels andto said plurality of dark pixel columns, the plurality of dark pixelrows having a width corresponding to a width of the active area ofpixels and the plurality of dark pixel columns, wherein each row of theplurality of dark pixel rows includes a first half of only opticallyblack pixels and a second half of only tied pixels.
 2. The imaging pixelarray of claim 1, wherein the optically black pixels and tied pixels inthe plurality of dark pixel rows are organized in a checkerboardpattern.
 3. The imaging pixel array of claim 1, wherein the first andthe second halves of adjacent dark pixel rows are located on alternatingsides of a vertical axis marking the midpoint of the dark rows.
 4. Theimaging pixel array of claim 1, wherein the optically black pixels andthe tied pixels in the plurality of dark pixel rows are organized in asymmetrical pattern.
 5. An imaging system, comprising: an imager,comprising a pixel array, comprising: an active area of pixels,comprising rows and columns of pixels; and a plurality of dark pixelsarranged in a plurality of dark pixel columns adjacent to a first sideof said active area of pixels and in a plurality of dark pixel rowsadjacent to a second side of said active area of pixels and to saidplurality of dark pixel columns, the plurality of dark pixel rows havinga width corresponding to a width of the active area of pixels and theplurality of dark pixel columns, wherein each row of the plurality ofdark pixels comprises optically black pixels and tied pixels; and ablack level calculating module that calculates a black level correctionvalue based on the outputs of an equal number of the optically blackpixels and the tied pixels.
 6. The system of claim 5, wherein the blacklevel calculating module further comprises: an averaging module tocalculate an average optically black pixel output and an average tiedpixel output for each row in the plurality of dark pixel rows; acomparing module to determine a difference between the average opticallyblack pixel output and the average tied pixel output for each row; and acalculating module to calculate the black level correction value as anaverage of the determined differences.
 7. The system of claim 5, whereinthe optically black pixels and tied pixels in the plurality of darkpixel rows are organized in a checkerboard pattern.
 8. The system ofclaim 5, wherein each row of the plurality of dark pixel rows isorganized into a first half and a second half, the first half comprisingoptically black pixels, and the second half comprising tied pixels. 9.The system of claim 8, wherein the first and the second halves of pixelsare located on alternating sides of adjacent dark pixel rows.
 10. Thesystem of claim 5, wherein the optically black pixels and the tiedpixels in the plurality of dark pixel rows are organized in asymmetrical pattern.
 11. An imaging pixel array, comprising: an activearea of pixels, organized as a plurality of rows and columns of pixels;and a plurality of dark pixels arranged in a plurality of dark pixelcolumns adjacent to a first side of said active area of pixels and in aplurality of dark pixel rows adjacent to a second side of said activearea of pixels and to said plurality of dark pixel columns, theplurality of dark pixel rows having a width corresponding to a width ofthe active area of pixels and the plurality of dark pixels columns,approximately half of said plurality of dark pixels in said dark pixelrows comprising optically black pixels and approximately half of saidplurality of dark pixels in said dark pixel rows comprising tied pixels,wherein the dark pixel rows comprise optically black pixels and tiedpixels arranged in a checkerboard pattern.